A film-covered battery is known, using a film as an exterior material. Generally, in a film-covered battery, a battery element, which is comprised of a positive plate, a negative plate, electrolyte, and the like, is housed in a sealed space formed of films.
In this regard, the sealed space is formed by thermally fusion-bonding opposing films around the battery element while it is surrounded by the films. Also, lead terminals are connected to the positive plate and negative plate of the battery element, respectively. These lead terminals protrude from the films.
As a film-covered battery is applied with a voltage which is out of a predefined range, gas is generated due to the electrolysis of an electrolyte solvent. The thus generated gas causes an increase in the inner pressure of the sealed space. Also, when the film-covered battery is used at high temperatures out of a predefined range, a material which causes gas is produced due to the decomposition of electrolytic salt and the like.
Basically, it is ideal to use a film-covered battery within a predefined range so as not to generate a gas. However, a large amount of gas can be sometimes generated within a sealed space when a control circuit for controlling a film-covered battery fails because of applying abnormal voltage to the film-covered battery, or when the temperature abnormally rises in surroundings due to some cause.
When a gas is generated in the sealed space, the inner pressure increases within the sealed space. This causes a rupture of a film or peeling of a fusion-bonded part of the films, and a gas is discharged from that part. Since it is impossible to identify where the film will burst and where the film will peel, devices located nearby can be adversely affected depending on where the film bursts or peels.
Patent Document 1 (JP-2005-203262A) describes a film-covered battery which is capable of discharging gas generated within the film-covered battery to the outside from a particular location of the film-covered battery.
This film-covered battery is provided with an inlet-shaped fusion-free zone continuous to a space which surrounds a battery element, which adjoins a fusion-bonding free area of films. As the inner pressure increases due to gas generated from the battery element, gas enters the fusion-free zone, and expands the exterior films of the fusion-bonding free area.
As the fusion-bonding free area expands, stress for tearing off the fusion-bonded part of the film concentrates in the fusion-bonded part adjacent to the fusion-free zone. As such, the film peels off progressively in this part in which the stress concentrates (hereinafter referred to as the “stress concentrating part”) ahead of the remaining fusion-bonding free area.
A pressure releasing part such as a hole is provided near the stress concentrating part. As the peeling advances in the stress concentrating part, the peeled area reaches the pressure releasing part. As the peeled area reaches the pressure releasing part, the interior of the film-covered battery communicates with the outside through the pressure releasing feature. In this way, the gas generated within the film-covered battery is discharged to the outside from the pressure releasing feature. Consequently, the gas can be discharged from a particular point (pressure releasing feature).
Patent Document 1: JP-2005-203262A